1. Field of the Invention
This invention relates to a superconductive element having a superconductive layer of a compound of a crystal structure of .beta.-W system (A-15 system) which is useful for superconductive devices, especially a cryotron, a superconductive switch and a superconductive memory. The invention relates also to a method for producing such superconductive element.
2. Brief Description of the Prior Art
A crystal structure of .beta.-W system is frequently observed in intermetallic compounds comprising a transition metal of a high melting point and a metal element of a low melting point, and many of these compounds having the .beta.-W crystal structure exhibit superconductivity and have a high superconductive critical temperature and a high critical current density even in a high magnetic field. As such superconductive material, there have been known binary intermetallic compounds of the .beta.-W crystal system such as V.sub.3 Ga, V.sub.3 Si, Nb.sub.3 Sn, Nb.sub.3 Al and Nb.sub.3 Ga, and ternary intermetallic compound of the .beta.-W crystal system such as Nb.sub.3 (Al,Ge) and Nb.sub.3 (Ga,Al).
The critical magnetic field at the liquid helium temperature (4.2.degree.K) is about 200 KG in the case of V.sub.3 Ga, about 250 KG in the case of V.sub.3 Si and Nb.sub.3 Sn, about 295 KG in the case of Nb.sub.3 Al, about 342 KG in the case of Nb.sub.3 Ga, and about 420 KG in the case of Nb.sub.3 (Al,Ge). In case a superconductive magnet generating a high magnetic field is prepared with use of such superconductive material, it is required to have not only a high critical magnetic field but also a high critical current density.
Compound type superconductors of conventional bulk materials have a very insufficient critical current density at 70 KG and in each case the critical current density at 70 KG is within a range of 10.sup.3 to 10.sup.4 A/cm.sup.2. Even in the case of Nb.sub.3 Al, the critical current density at 70 KG is about 5 .times. 10.sup.3 A/cm.sup.2.
When a superconductive compound layer is formed according to the diffusion method employing a diffusion temperature not exceeding 1,200.degree.C., each of the compounds V.sub.3 Si, V.sub.3 Ga and Nb.sub.3 Sn exhibits at 70 KG such an excellent critical current density that it exceeds 10.sup.5 A/cm.sup.2. However, in the case of V.sub.3 Si, V.sub.3 Ga and Nb.sub.3 Sn the critical magnetic field is low and in about 200 to 250 KG at the liquid helium temperature, and it is not comparable to the critical magnetic field of Nb.sub.3 Al or the like, that exceeds about 300 KG.
In the case of materials of the Nb-Al, Nb-Ga, Nb-Al-Ge and Nb-Ga-Al systems having a high critical magnetic field, however, in order to form a .beta.-W crystal phase according to the diffusion method, the diffusion temperature should be heightened to 1,500.degree. to 1,600.degree.C. or higher. If the diffusion temperature is lower than the above level, in the case of a material comprising Nb and Al, a compound of a higher Al concentration such as Nb.sub.2 Al or NbAl.sub.3 is formed and in the case of a material comprising Nb and Ga, a compound of a higher Ga concentration such as Nb.sub.5 Ga.sub.3 is formed. Thus, in order to form a layer of Nb.sub.3 Al or the like by the diffusion method, it is necessary to employ a diffusion temperature exceeding 1,500.degree. to 1,600.degree.C. For this reason, in such cases a drastic reduction of the critical current density cannot be avoided.
In a compound type superconductive material, pinning points of magnetic fluxes reside in the grain boundary, and it is known that as the grain size is smaller, the number of pinning points increases and hence, the critical current density is heightened. The grain size of such compound is larger as the preparation temperature is higher. Accordingly, in order to increase the critical current density it is preferred that the preparation temperature is as low as possible.
In case a film material is prepared according to the CVD method, the method of the simultaneous spattering of each element or the method of the simultaneous vacuum deposition of each element, distances between different atoms scattered on a substrate are short, and therefore, it is expected that the diffusion temperature, i.e. the substrate temperature at the film preparation, can be made lower than the above-mentioned bulk diffusion temperature. In fact, it was reported that according to the CVD or spattering method, the temperature for preparation of Nb.sub.3 (Al,Ge) can be made lower than 1,000.degree.C. and the critical current density can be increased. In this case, however, the critical temperature is as low as 14.degree.K or 10.7.degree.K. If according to the remaining method, i.e., the method of the simultaneous vacuum deposition of each element, it is possible to increase the critical current density in materials of Nb-Al, Nb-Ga, Nb-Al-Ge and Nb-Ga-Al systems without lowering the critical temperature below that of the bulk aging material, it will be possible to prepare with use of these materials superconductive magnets capable of generating a higher magnetic field than that attainable in such materials as Nb.sub.3 Sn, V.sub.3 Ga and V.sub.3 Si, for example, superconductive magnets capable of generating a magnetic field of more than 200 KG.
However, as detailed hereinbelow, if a conventional substrate is employed, only a superconductive film of a relatively low critical current density and a relatively low critical temperature is obtained.
As a substrate for a superconductive film of a .beta.-W crystal system compound, there have heretofore been used such insulating materials as molten quartz and such metallic materials as Nb and W.
In a case a film of a .beta.-W crystal system compound, for example, a Nb-Al film having a .beta.-W crystal structure and a critical temperature approximating that of the bulk is prepared, it is necessary that the simultaneous vacuum evaporation should be conducted at a substrate temperature exceeding 800.degree.C., or that after the simultaneous vacuum evaporation is conducted at a substrate temperature approximating room temperature, the vacuum heat treatment should be conducted at about 1,000.degree.C. Also, in the case of a superconductive layer of the Nb-Al-M ternary system in which a part of Al in Nb-Al is replaced by other metal, and in the case of a superconductive film of the Nb-Ga or Nb-Sn binary or ternary system, the preparation temperature should be made higher than about 800.degree. to about 900.degree.C.
Fused quartz can be used at a temperature as high as 1,100.degree. to 1,200.degree.C., but in some film-constituting elements, it reacts with the film at lower temperatures, with the result that it is made difficult to form a compound of the .beta.-W crystal system. For instance, in the case of a Nb-Al film, if the Nb : Al atomic ratio is less than 3 : 1 and the temperature is higher than about 900.degree.C., the reaction between Al and a quartz substrate is so vigorous that both the critical temperature and the critical current density are lowered. Further, in case fused quartz is employed as a substrate, when the preparation temperature exceeding 900.degree.C. is lowered to room temperature by cooling, because of the difference of the thermal expansion coefficient between fused quartz (0.35 .times. 10.sup..sup.-6 /.degree.C. at room temperature) and the film (8 .times. 10.sup..sup.-6 /.degree.C. at room temperature in the case of Nb.sub.3 Al,) cracks are readily formed and in extreme case the film is peeled from the substrate.
In case a heat-resistant metal such as Nb and W is employed as a substrate, it is possible to use such substrate at a temperature of up to about 2,000.degree.C. Accordingly, the substrate can resist the preparation temperature of about 1,000.degree.C. However, when the .beta.-W crystal structure of the film is unstable, the substrate reacts with film-constituting elements. For instance, when a film of the Nb-Al-Cu system formed on a Ta substrate by simultaneous vacuum deposition is subjected to the vacuum heat treatment at 900.degree.C., the .beta.-W system compound is decomposed and an alloy of Nb and Ta is formed. Further, since the above substrate metal has a better conductivity at low temperatures than the prepared film, electric insulation cannot be attained between the film and substrate and therefore, it is difficult to determine the electric characteristics of the film. Further, in case a metallic material is used as a substrate, as in the case of fused quartz, because of the difference of the thermal expansion coefficient between the superconductive film and the substrate, formation of cracks in the film cannot be avoided, and therefore, use of such substrate is not preferred.
The above explanation has been made mainly with reference to the Nb-Al film, but the same holds true in the case of the superconductive films of the Nb-Ga, Nb-Al-Ge and Nb-Ga-Al system compounds.
As described hereinabove, superconductive elements according to the conventional techniques exhibit only an insufficient critical current density in either a low magnetic field or a high magnetic field or both of these fields, and in some cases, cracks are formed in the films or film peeling is observed.